ter pollution in the Demon Realm, Leo and Mernes head off to investigate Lake Amia and its resident water spirit, Undine. But one thing stands in their way--the legendary "Demon King Slayer," who swears to protect Undine at all costs! Just who is this mysterious foe?! Meanwhile, Demon Queen Echidna puts her plan into motion, donning a perfect disguise to confer with Leo about her troubles...